Electron beam generating system for high beam potentials

ABSTRACT

This invention concerns an electron beam generating system for high beam voltages in which the insulation between a grounded casing and the high-voltage electrodes is achieved by an insulating body surrounding the high-voltage electrodes with a small distance, so that the dimensions of the system are considerably smaller than with arrangements in which the insulation is primarily achieved by providing sufficient vacuum space about the high-voltage electrodes. The invention is further concerned with an automatically actuable replacement device for cathode holders, said replacement device being adapted to be controlled by monitoring means for at least one beam parameter so that the cathode holder under operation will be automatically replaced by a fresh cathode holder and cathode when the magnitude of the sensed beam parameter falls outside a predetermined range.

O United States tet [72] Inventor Karl-Heinz Steigerwald Mozartstrrisse 27, Lochham near Munich, Germany [21] Appl. No, 601,082 [22] Filed Dec.l2,1966 [45] Patented Aug. 24, 1971 [32] Priority Dec. 13, 1965 [33] Germany [31 ST 24757 [54] ELECTRON BEAM GENERATTNG SYSTEM FOR HIGH BEAM POTENTIALS 5 Claims, 3 Drawing Figs.

[52] 11.5. C1 313/82, 313/84, 313/251, 250/495, 313/237 [51] Int. Cl ..lHl01j 29/00, H01 j 5/ 14 [50] Field of Search 3l3/82,84: 250/495 [56] References Cited UNITED STATES PATENTS 2,305,458 12/1942 Ruska et a1. 313/82 2,443,916 6/1948 313/82 2,984,762 5/1961 313/84 X 3,155,827 11/1964 Nixon 250/49.5-1

Primary Examiner- Robert Segal Attorney-Sandoe, Hopgood & Calimafde ABSTRACT: This invention concerns an electron beam generating system for high beam voltages in which the insulation between a grounded casing and the high-voltage electrodes is achieved by an insulating body surrounding the highvoltage electrodes with a small distance, so that the dimensions ofthe system are considerably smaller than with arrangements in which the insulation is primarily achieved by providing sufficient vacuum space about the high-voltage electrodes.

The invention is further concerned with an automatically actuable replacement device for cathode holders, said replacement device being adapted to be controlled by monitoring means for at least one beam parameter so that the cathode holder under operation will be automatically replaced by a fresh cathode holder and cathode when the magnitude of the sensed beam parameter falls outside a predetermined range Ill/I;

ELECTIETUN BEAM GENERATING SYSTEM IFUM ll-ill Gll-ll IESIEAM IPOTIENTIIALS The invention concerns an electron beam generating system for high beam potentials.

In known electron beam generating systems for high beam potentials the high tension parts of the electron beam generating system are isolated by vacuum or insulators from the parts connected to earth potential or accessible from the outside. More particularly the distance between the high tension conducting parts and the metal vacuum housing is at all points so great that an electric flashover to the housing is reliably prevented. Holders for high tension parts are mounted in insulators having a large surface creep distance between these holders and the earthed parts on which the insulators are mounted. In known electron beam generating systems of this type, when using high beam voltages of the order of magnitude of 100 kv., large insulators with long leakage paths are necessary and therefore such apparatus tends to be large and clumsy in use. Particularly disturbing is the large dimensions of the housing to give the necessary distances between the electron beam generating system and the internal wall surfaces of the housing.

A further disadvantage of known electron beam generating systems consists in that changing the cathode involves cumbersome and time wasting work; meanwhile the electron beam generating system is switched off and is not performing any useful work.

When the electron beam generating system has been switched on again, a considerable waiting period is required until a stable thermal and electric (more particularly electrostatic) state of working cycle conditions is obtained and the electron beam no longer shows any undesired changes of its dimensions or power.

Both the said large housing dimensions and the cumbersome and inconvenient method of cathode removal have hitherto been an obstacle to wider technological application of electron beam generating systems.

One of the objects of the invention is to provide an electron beam generating system of the kind referred to above which, compared with known installations, whilst retaining a high beam potential has considerably reduced dimensions, and more particularly a considerably reduced diameter.

According to the invention an electron beam generating system using high beam potentials has the high tension conducting parts of the electrode assembly enclosed in a chamber the external surface of which is covered by an earthed electric conductor and the internal wall surfaces of which are formed from an insulating body and enclose the high tension carrying parts, the thickness of insulation provided being determined, at least in some parts, partially or wholly by the dielectric strength of the insulating material.

In an electron beam generating system in accordance with the invention the use of this highly stressed insulating body in the path between the high tension carrying parts and the parts connected to earth potential, provides a considerable reduction of insulator size. In contrast to known structures of electron beam generating systems, the electron beam generating system in accordance with the invention is formed not only from the aspect that an as large as possible a creep or leakage distance must exist on the surface of the insulating material parts, but also that the potential gradient through the insulation is an appreciable fraction of the dielectric strength of the material.

A further object of the invention consists in that an electron beam generating system of the kind referred to above is provided, in which a cathode change may be effected rapidly and simply, so that the electron beam generating system operation is interrupted for only a short time.

According to a feature of the invention a magazine device to receive spare cathodes and for automatically feeding them to the beam generating system whilst under vacuum is provided in a space located behind the cathode system of the electron beam generating system. This magazine device is preferably so controlled by a beam data supervising device that a cathode change takes place automatically when the supervised beam data depart to a predetermined degree from a given value. The magazine device may be so formed that during the cathode change the beam voltage need not be switched off; because of this construction the time required for the cathode change has been drastically reduced.

Such electron beam generating systems equipped with automatic magazine devices able to operate automatically under a vacuum without interrupting the high-tension supply, need no assitance from an operator during the cathode change. Furthermore the out-of-action period is so short that during this period no serious changes occur in the thermal and/or electric characteristic data of the beam generating system and a workpiece possibly being processed (cooling off etc.). Since the high tension is not switched off, the waiting period required after a cathode change necessary in known apparatus to stabilize the temperature and charge distribution to the operating state, is completely avoided.

With a beam generating system in accordance with the invention which has a magazine device operating under vacuum and high tension it is possible to reduce the time required for the cathode change to a few seconds.

According to a further feature of the present invention the electron beam generating system is so formed that the electric center of the creep or leakage distance on the surface of the insulating body facing the electrode system is opposite the electric center of the electron acceleration path. This structure of the electron beam generating system causes the elec tric loading of the insulating body at least in the proximity of the electric center of the accelerating distance to amount to only half the overall potential difference between the high tension carrying and earthed parts.

According to a further feature of the present invention the insulating body is so formed that the surface potentials determined by the leakage distance on the surface of the insulating body facing the electrode system, substantially coincide with the potentials determined on this surface by the geometry of the metal parts. This configuration of the insulating body causes the influences which are exerted on the beam geometry with possible changes of the surface properties of the insulating body, to be substantially reduced.

Further features and advantages of the invention will be seen from the description below in combination with the accompanying drawings, in which:

FIG. ii shows schematically an axial section through the upper part of an electron beam generating system in accordance with the invention.

FIG. 2 shows a view similar to FIG. l, but including a cathode magazine with cathode changing device.

FIG. 3 is an enlarged view of the insulating body in axial section.

An electron beam generating system is shown in FIG. II and includes a vessel il, which is part of the vacuum housing and comprises a hollow cylindrical insulating body 5, on the external surface 2 of which an earthed electric conductor 3 in the form of a sheet metal envelope is provided. The envelope 3, may be slipped onto the insulating body 5 and fixed by means of tightening straps 30.

A cathode system is arranged in the interior of the substantially hollow cylindrical vessel ll, including all high tension carrying parts; the inner wall 4 of the vessel is relatively close to the cathode system, and the insulation thickness between the cathode system and the metal envelope 3 is substantially determined by the dielectric strength of the insulator. The cathode system is secured by means of a supporting ring 6 to the insulating body 5. The supporting ring 6 may be fused in the insulating body during its manufacture. For this purpose the insulating has thickened portions 35 at the securing points of the supporting ring 6 in which the edge portions 35 at the securing points of the supporting ring 6 in which the edge portions 34 of the supporting ring 6 formed as anchoring members are located, the said edge portions being reduced for electrical reasons. The distance between the edge portions 34 and the insulating body 5 is selected to be so large that the heat from the cathode system during operation passing via the supporting ring 6 is unable to heat the insulating body 5 to such an extent that the material of which the insulating body 5 is made is impaired in its properties. The cathode system in the embodiment shown comprises a Wehnelt electrode 7 in the control aperture of which a V-shaped cathode 8 is located. The cathode 8 is secured in a cathode holder 32 which engages in guide tracks (not shown) of the Wehnelt electrode 7 by meansof lateral insulating extensions 36 in a predetermined position relative to the Wehnelt electrode. On the upper end the cathode holder 32 is provided with anchoring means which are adapted to engage with a key (not shown) for removing and inserting the cathode holder. In the embodiments as shown in FIGS. 1 and 2 simple bayonet slots 33 are indicated as anchoring means; other devices may be used to produce an engagement between the cathode holder and a key.

Pump openings 9 are formed in the supporting ring 6 to equalize the pressure in the spaces 14 and 15 located above and below the cathode system.

As shown in FIGS. 1 and 2, the insulating body 5 acts as part of the vacuum housing. The upper end of the vessel 1 is provided with a detachable lid 12 which is also made of insulating material and has a shoulder 16 fitting the inside surface of the insulating body 5, and a metal cover 13. The cover 13 is screwed to the lid 12 by means of screws 29. Sealing between the insulating body 5 and the lid 12 is effected by means of an annular packing ring 17.

The lower end of the insulating body 5 is provided with a flange 25 and is fixed by this flange 25 to an anode plate 11 and to a flange 26 of a further housing part 20, by means of screws 27. To ensure accurate alignment of the vessel 1, the anode ring 11 and the lower housing portion fixing pins 24 may be provided.

Sealing between the said parts is effected by annular packings 23 and 22. The parts adjoining the lower housing portion 20 and those accommodated in this housing are not shown in the figures. These may be electron-optical systems, and apparatus for beam deflection, and material treatment.

The anode plate 11 has openings 18 and a central anode projection 10. Directly below the anode plate'll a diaphragm system (not shown) may be accommodated, this being mounted in the chamber 19, enclosed by the housing portion 20, and may prevent evaporated material from passing from the treatment chamber into the chambers 14 and 15 of the electron beam generating system. The vessel 1 is evacuated by a pump connection established to chamber 19 or one of the adjoining chambers.

As evident from FIGS. 1 and 2, the cathode system divides the interior of the insulating body 5 into a rear chamber 14 and a front chamber 15; access to the rear chamber 14 may be obtained by removing the lid 12. Form tools, cathodes and other parts may be introduced through the aperture into the interior of the beaker-shaped Wehnelt electrode 7 which is open at the top.

In FIGS. 1 and 2 the electric center of the electron accelera' tion path between the cathode 8 and the anode 10 is indicated by a broken line 28. In the arrangement shown in the figures, this line 28 intersects the inner surface 4 of the insulating body 5 substantially half way between the supporting ring 6 and the anode ring 1 1, so that the electrical center of the acceleration distance lies substantially opposite the electrical center of the leakage distance between the supporting ring 6 connected to high tension and the anode ring 11 connected to earth potential. The surface potentials formed during operation on the inner surface 4 of the insulating body 5 influence the position of the electron beam produced. In the embodiments shown in FIGS. 1 and 2 the charge distribution on the surface 4 is substantially constant, since the leakage potentials and geometric potentials coincide.

The high operating voltage to the cathode system is supplied via leads (not shown), which traverse the insulating body substantially radially are terminated on a socket mounted on the outer jacket 3, to which feeder lines are connected.

The embodiment of an electron beam generating system shown in FIG. 2 differs from the embodiment shown in FIG. 1 by the fact that a further chamber 21 is joined to the rear of chamber 14 in which there is a magazine device containing spare cathodes, with means for feeding the spare cathodes sequentially to the beam generating system under vacuum. In this embodiment the aperture 16 of the rear chamber 14 is open, and instead of the lid 12 a metal ring 51 is mounted on the upper surface of the insulating body 5. This metal ring is sealed to the insulating body 5 by an annular packing 57. The upper end of the insulating body 5 is provided with a flange 54, which is screw connected with the edge of the metal ring 51 and a flange 55 of the chamber 21 mounted thereon by means of screws 55. The chamber 21 is sealed by means of an annular packing 56 to the metal ring 51, which is substantially cylindrical and comprises a jacket 58 rising from the flange 55, and a lid 93. Fixing pins 52 are used for aligning the parts 5, 51 and 58.

The magazine device to receive spare cathodes and feed them to the beam generating system contains a storage for cathode holders ready for operation and cooperating transfer device for changing an operative cathode holder 32 for one from the magazine. In the embodiment shown in FIG. 2 the cathode store consists of a circular magazine disc 64 which has a bearing 63 rotatable about a pivotal axis extending parallel to the beam axis and having several magazine openings 65 spread over its circumference, in which openings cathode holders 132 are detachably secured. The outer edge of the magazine disc 64 is provided with a toothed ring which meshes with a gearwheel 67 mounted in the bearing 62. The gearwheel 67 is driven by a pinion 68 which is fixed to the driving shaft of a driving motor 69 located in chamber 21. The driving motor 69 is carried on supports 70 and is connected to external terminals by means of lead-in wires 71 and a vacuum seal 72, which is cemented into the lid 93. In the embodiment shown in FIG. 2 the transfer device is provided with a pushrod 77 which is passed vacuumtight through the center of the lid 93 in the beam direction by means of a packing 75. The packing is inserted in a ring 73 and retained in situ by a pressure collar 74 which is secured by screws 76. The lower end of the pushrod 77 is provided with an engaging means fitting the anchoring elements 33 of the cathode holder 32, the means consisting of a radial pin 78 which engages with the bayonet holder 33. By lowering the pushrod 77, engaging the pin 78 in the bayonet slot 33, twisting the pushrod 77 and lifting the pushrod 77, a cathode holder 32 is located in the Wehnelt electrode 7 may be pulled out, upwards. In FIG. 2 this state is shown. To permit this pulling-out through the magazine disc 64, the latter has at least one large opening 65 through the which the cathode holder 32 passes with clearance. On reaching the position shown in FIG. 2, the magazine disc 64 is rotated by switching on the driving motor 69 to such an extent that an empty magazine opening 66 as sumes a position exactly below the cathode holder 32, Lowering the pushrod 77 permits this cathode holder to be inserted in the magazine opening, anchored there, and released from the pushrod by means of a suitable movement of the pushrod 77. The pushrod may then be removed upwardly and the magazine disc rotated to such an extent by renewed switching on of the driving motor 69 to cause a full magazine opening 66, i.e. the magazine opening filled in FIG. 2 with the spare cathode holder 132, to assume a position exactly below the pushrod 77. The switching of the driving motor 69 to on" and ofi may be effected by means of limit switches or like known devices, (not shown). When the replacement cathode holder 132 is located accurately below the pushrod 77, it is engaged and drawn upwards by the pushrod 7. The magazine disc 64 is then rotated again to such an extent that the large opening 65 assumes a position beneath the spare cathode holder, and the pushrod S5 is moved downwards until the spare cathode holder secured thereto is inserted in the Wehnelt electrode 7. Twisting the pushrod 77 disengages it from the spare cathode holder whereafter it is moved upwards again. To permit observation of this replacement operation the jacket 58 or the lid 93 has an inspection window 59 formed therein. Filling and emptying the magazine disc 64 with cathode holders is effected via an opening formed in the lid 93 which is closed by a lid 60 and sealed by a packing 61. The movements of the rod 77 during the cathode change may be automated and coordinated with the movements of the magazine disc 64.

Such apparatus may take various forms. Thus FIG. 2 indicates that a central section of rod 77 is provided with a thread; on this central section an internally toothed sleeve 94 is mounted. The sleeve also has an external toothed ring which meshes with a worm wheel 80 of a driving motor 81. The sleeve is mounted so as to be vertically nondisplaceable between stops 95. The stops 95 and motor 81 are mounted on a pedestal bearing 86 and a frame 83. The upper end of the rod 77 supports a control cylinder 8d which is provided with a control slot 85 and a stop means 96. Engaging in the curve is a fixed control pin, not shown, and the stop means in the end positions of the vertical movement of rod 77 engage with limit switches 82. The control slot 85 extends in such a way that, when the driving motor 81 rotates, the necessary propelling and rotary movements of the rod 77 for releasing and engaging the cathode holders result automatically at the right positions and to the correct extent. The design of this automatic control is not dealt with further herein.

The pushrod 77, at least in its lower portion, is made of an electrically insulating material. Whilst it will normally be ensured that when a cathode change procedure is in operation the high beam acceleration voltage is switched off, such a pushrod made at least partly of insulating material may be used for changing the cathode without switching off the beam acceleration potential. This is particularly advantageous when continuously processing workpieces with electron beams, since immediately after inserting the new cathode the process may be continued and it is not necessary to wait as in the case of a temporary switching-off of the beam acceleration voltage until the electric field, which was at least in part determined by the surface charges on the insulators, again becomes stationary. The cathode change may take place at such a high rate that, for example, in electron beam welding the thermal conditions prevailing at the welding point hardly change during the cathode change.

A cathode change operation is preferably initiated automatically as soon as a supervised beam parameter, eg the emission current or the beam diameter, deviates from a predetermined range of tolerances or a nominal value. Due to the automatic change of cathode then occurring it is ensured that serious disturbances of the beam originating from the aging of the cathode are eliminated. If the supervised beam parameter still lies outside the nominal value range after the cathode change, the cathode is preferably not changed again, but an alarm signal is given to draw the operators attention to the fact that a breakdown has occurred which is not to be attributed to the natural aging of the cathode. Measuring, supervising and control instruments required for this mode of operation may be designed by any expert in the field of measuring and control technology; such devices are therefore not described in detail herein.

FIG. 3 shows that the leakage path on the inside surface 41 of the insulating body 5 may be extended by ridges 9i and 92 in the proximity of the supporting ring 6. Such or similar measures enable the surface potentials on the insulating body determined by the leakage path and facing the electrode system to coincide substantially with the potentials deter mined by the geometry and location of the metal parts. These steps ensure that mutual influences between the electrode beam and the charge distribution on the surface 5 are reduced to a minimum.

What I claim is:

1. An electron beam generating system for high beam accelerating voltages comprising:

a hollow evacuable chamber having supported therein an electrode system which includes a cathode assembly and an anode;

said cathode assembly including a cathode, a control electrode mounted adjacent the cathode, and supporting structure supporting the control electrode and the cathode in the chamber;

the cathode assembly carrying high voltage with respect to ground potential and the anode being held substantially at ground potential;

said chamber comprising a hollow insulating body forming at least a part of the interior walls of the chamber and being covered on its outer surface with an earthed electric conductor, the distance between the earthed conductor and the cathode assembly being, at least in some areas, smaller than the insulating distance which would be necessary in the absence of the insulating body;

the minimum thickness of the insulating body between the cathode assembly and the earthed conductor being determined by the dielectric strength of the insulating material of which the insulating body is made;

said chamber having its axis substantially coinciding with the axis of the beam to be generated, the cathode assembly being supported at a predetermined axial position in an inner circumferential area of the cylinder and the anode having a central anode opening through which the generated beam passes out of the cylinder;

the cathode assembly and the anode being spaced apart and having central portions, respectively, defining therebetween an acceleration distance for the electron beam generated;

the interior surface of the chamber being shaped so that the surface potentials created thereon by the creep current between the inner circumferential area where the cathode assembly is supported and the anode substantially coincide with the potential distribution generated by the electric field between the cathode assembly, the anode and the earthed electric conductor covering on the outer surface of the insulating body; and

said chamber having an access opening normally closed by detachable closure means comprising a lid made of insulating material with an outer conducting covering in contact with said earthed conductor cover when the lid is in position closing the access opening.

2. The system of claim ll wherein the cathode assembly, the anode, the chamber and the insulating body are adapted for operating with a voltage in the order of at least kilovolts.

3. The system of claim 1 wherein the hollow insulating body is cylindrical.

4. The system of claim ll, wherein the midpoint of the acceleration distance is located substantially at an axial position in the chamber which substantially corresponds to half the length of a line drawn in the axial plane :in the chamber along the interior surface of the chamber between the inner circumferential area where the high voltage carrying parts are supported and the anode.

5. The system of claim 41 wherein at least a portion of the interior surface of the chamber is corrugated. 

1. AN ELECTRON BEAM GENERATING SYSTEM FOR HIGH BEAM ACCELERATING VOLTAGES COMPRISING: A HOLLOW EVACUABLE CHAMBER HAVING SUPPORTED THEREIN AN ELECTRODE SYSTEM WHICH INCLUDES A CATHODE ASSEMBLY AND AN ANODE; SAID CATHODE ASSEMBLY INCLUDING A CATHODE, A CONTROL ELECTRODE MOUNTED ADJACENT THE CATHODE, AND SUPPORTING STRUCTURE SUPPORTING THE CONTROL ELECTRODE AND THE CATHODE IN THE CHAMBER; THE CATHODE ASSEMBLY CARRYING HIGH VOLTAGE WITH RESPECT TO GROUND POTENTIAL AND THE ANODE BEING HELD SUBSTANTIALLY AT GROUND POTENTIAL; SAID CHAMBER COMPRISING A HOLLOW INSULATING BODY FORMING AT LEAST A PART OF THE INTERIOR WALLS OF THE CHAMBER AND BEING COVERED ON ITS OUTER SURFACE WITH AN EARTHED ELECTRIC CONDUCTOR, THE DISTANCE BETWEEN THE EARTHED CONDUCTOR AND THE CATHODE ASSEMBLY BEING, AT LEAST IN SOME AREAS, SMALLER THAN THE INSULATING DISTANCE WHICH WOULD BE NECESSARY IN THE ABSENCE OF THE INSULATING BODY; THE MINIMUM THICKNESS OF THE INSULATING BODY BETWEEN THE CATHODE ASSEMBLY AND THE EARTHED CONDUCTOR BEING DETERMINED BY THE DIELECTRIC STRENGTH OF THE INSULATING MATERIAL OF WHICH THE INSULATING BODY IS MADE; SAID CHAMBER HAVING ITS AXIS SUBSTANTIALLY COINCIDING WITH THE AXIS OF THE BEAM TO BE GENERATED, THE CATHODE ASSEMBLY BEING SUPPORTED AT A PREDETERMINED AXIAL POSITION IN AN INNER CIRCUMFERENTIAL AREA OF THE CYLINDER AND THE ANODE HAVING A CENTRAL ANODE OPENING THROUGH WHICH THE GENERATED BEAM PASSES OUT OF THE CYLINDER; THE CATHODE ASSEMBLY AND THE ANODE BEING SPACED APART AND HAVING CENTRAL PORTIONS, RESPECTIVELY, DEFINING THEREBETWEEN AN ACCELERATION DISTANCE FOR THE ELECTRON BEAM GENERATED; THE INTERIOR SURFACE OF THE CHAMBER BEING SHAPED SO THAT THE SURFACE POTENTIALS CREATED THEREON BY THE CREEP CURRENT BETWEEN THE INNER CIRCUMFERENTIAL AREA WHERE THE CATHODE ASSEMBLY IS SUPPORTED AND THE ANODE SUBSTANTIALLY COINCIDE WITH THE POTENTIAL DISTRIBUTION GENERATED BY THE ELECTRIC FIELD BETWEEN THE CATHODE ASSEMBLY, THE ANODE AND THE EARTHED ELECTRIC CONDUCTOR COVERING ON THE OUTER SURFACE OF THE INSULATING BODY; AND SAID CHAMBER HAVING AN ACCESS OPENING NORMALLY CLOSED BY DETACHABLE CLOSURE MEANS COMPRISING A LID MADE OF INSULATING MATERIAL WITH AN OUTER CONDUCTING COVERING IN CONTACT WITH SAID EARTHED CONDUCTOR COVER WHEN THE LID IS IN POSITION CLOSING THE ACCESS OPENING.
 2. The system of claim 1 wherein the cathode assembly, the anode, the chamber and the insulating body are adapted for operating with a voltage in the order of at least 100 kilovolts.
 3. The system of claim 1 wherein the hollow insulating body is cylindrical.
 4. The system of claim 1, wherein the midpoint of the acceleration distance is located substantially at an axial position in the chamber which substantially corresponds to half the length of a line drawn in the axial plane in the chamber along the interior surface of the chamber between the inner circumferential area where the high voltage carrying parts are supported and the anode.
 5. The system of claim 4 wherein at least a portion of the interior surface of the chamber is corrugated. 